Our understanding of the world relies heavily on our senses, and the sense of sight is arguably one of the most crucial. It allows us to perceive the vast array of objects and phenomena around us, from mountains and rivers to trees, people, celestial bodies like the moon and stars at night, and even the words printed on a page.

But how exactly is seeing made possible? What process allows us to visually perceive the objects around us?

What Makes Things Visible

While it might seem intuitive that our eyes simply "see" objects, the process is more complex. Can you see anything in a completely dark room? No. This indicates that eyes alone are not sufficient for vision.

We are able to see objects because light from the object enters our eyes. This light might be emitted by the object itself (if it's a source of light), or it might be light that has fallen on the object and been reflected from its surface.

For instance, the Sun, fire, candle flames, and electric lamps are objects that produce their own light; they are called luminous objects. We see them when the light they emit directly enters our eyes.

Most objects we see, however, do not produce their own light. We see them because they reflect light that falls on them from a luminous source. The Moon, for example, does not produce light; we see it because it reflects sunlight. Objects that are visible due to the reflection of light from other sources are called illuminated objects.

A polished or shiny surface, like a mirror, is particularly effective at changing the direction of light that strikes it. This phenomenon is called reflection.

Laws Of Reflection

The way light reflects from a surface follows specific rules or laws. To understand these laws, we can perform experiments using a light source, a mirror, and a surface to observe the light rays.

When a ray of light strikes a surface, it is called the incident ray. The ray that bounces back from the surface after striking it is called the reflected ray.

An ideal 'ray' of light is a theoretical concept; in reality, light travels in beams made of many rays. For simplicity in diagrams, we often represent a narrow beam as a single ray.

To precisely describe reflection, we draw a line perpendicular to the reflecting surface at the point where the incident ray strikes. This line is called the normal to the reflecting surface at that point.

The angle between the normal and the incident ray is called the angle of incidence ($\angle i$). The angle between the normal and the reflected ray is called the angle of reflection ($\angle r$).

By measuring these angles for various angles of incidence, we discover the first law of reflection:

Law 1: The angle of incidence is always equal to the angle of reflection ($\angle i = \angle r$).

The second law of reflection relates the orientation of the incident ray, reflected ray, and the normal:

Law 2: The incident ray, the reflected ray, and the normal to the reflecting surface at the point of incidence all lie in the same plane.

This means if you imagine a flat surface (a plane) containing the incident ray and the normal at the point of incidence, the reflected ray will also be on that same flat surface. You can demonstrate this by using paper that can be bent, showing that the reflected ray disappears if the paper is bent out of the plane containing the incident ray and normal.

If a ray of light is incident perpendicular to the surface (i.e., along the normal), the angle of incidence is $0^\circ$. According to the first law, the angle of reflection will also be $0^\circ$. This means the reflected ray will travel back along the same path as the incident ray.

These laws govern how light reflects from all types of surfaces.

Regular And Diffused Reflection

The appearance of reflected light depends on the nature of the reflecting surface.

• Regular Reflection: This occurs when a beam of parallel light rays strikes a smooth, polished, and regular surface (like a plane mirror). According to the laws of reflection, each individual ray obeys $\angle i = \angle r$. Since the surface is smooth and the normal is parallel for all incident rays, the reflected rays are also parallel to one another. Images are formed by regular reflection.
• Diffused or Irregular Reflection: This occurs when a beam of parallel light rays strikes a rough or irregular surface (like a cardboard surface or chalk powder). Even though the laws of reflection ($\angle i = \angle r$) are still valid at each point on the surface, the irregularities mean that the normal is oriented differently at different points. As a result, the parallel incident rays are reflected in various different directions, and the reflected rays are not parallel to one another.

  Diffused reflection is NOT a failure of the laws of reflection; the laws are followed at every point. The diffusion is caused by the unevenness of the surface.

Most objects we see are visible due to diffused reflection of light from their surfaces. We see a book, a table, or a wall because they scatter the light falling on them in all directions. A mirror is an exception, where regular reflection allows us to see a clear image.

Reflected Light Can Be Reflected Again

Light that has been reflected from one surface can strike another surface and be reflected again. This concept is fundamental to how we see images in multiple mirrors.

When you go to a hairdresser, you sit in front of a mirror. To see the back of your head, the hairdresser holds another mirror behind you. The light from the back of your head travels to the mirror held behind you, reflects off it, travels to the mirror in front of you, reflects off that mirror, and finally enters your eyes. This second reflection allows you to see the image of the back of your head.

A periscope, which allows you to see objects over an obstacle (like over a wall or from a submarine), also works on the principle of double reflection using two plane mirrors.

Multiple Images

A single plane mirror forms a single image of an object. However, when two or more plane mirrors are used together, they can form multiple images due to repeated reflections.

The number of images formed depends on the angle between the two mirrors.

• When two plane mirrors are placed at a right angle ($90^\circ$) to each other, you can see three images of an object placed between them.
• If the angle between the mirrors is changed, the number of images also changes. The number of images can be calculated using the formula:

  $n = \left( \frac{360^\circ}{\theta} \right) - 1$

  where $n$ is the number of images and $\theta$ is the angle between the mirrors. (This formula applies when $\frac{360^\circ}{\theta}$ is an integer).
• When two plane mirrors are placed parallel to each other ($0^\circ$ angle), an infinite number of images are formed. Light reflects back and forth between the mirrors repeatedly.

Kaleidoscope

The principle of multiple reflections formed by mirrors placed at an angle is used in a toy called a kaleidoscope. A kaleidoscope uses three rectangular mirror strips joined to form a prism, placed inside a tube. Pieces of coloured glass or bangles are placed at one end. Looking through a hole at the other end, one sees beautiful, intricate patterns formed by the multiple reflections of the coloured pieces in the mirrors.

A key characteristic of a kaleidoscope is that every time you rotate the tube, the coloured pieces rearrange, creating a new and unique pattern. This makes it a source of inspiration for designers and artists.

Sunlight — White Or Coloured

Sunlight, which appears white to us, is actually composed of multiple colours. You may have learned in Class VII that white light is made up of seven constituent colours (Violet, Indigo, Blue, Green, Yellow, Orange, Red - VIBGYOR).

This can be demonstrated by passing sunlight through a prism or using a mirror and water setup. When sunlight passes through a prism (or acts like one, as with a mirror in water), it is split into its seven component colours. This phenomenon of splitting white light into its constituent colours is called dispersion of light.

A natural and beautiful example of dispersion is the formation of a rainbow, which occurs when sunlight passes through raindrops (acting as tiny prisms) in the atmosphere.

What Is Inside Our Eyes?

Our eyes are complex and vital sense organs that allow us to see. We see objects when light reflected or emitted from them enters our eyes. Understanding the structure and function of the eye is crucial for appreciating how vision works and how to care for our sight.

The human eye is roughly spherical in shape and has several important parts:

• Outer Coat: The tough, white outer layer of the eye is called the sclera. It provides protection to the internal parts.
• Cornea: The transparent front part of the eye, which is a bulge in the outer coat. Light enters the eye through the cornea.
• Iris: Located behind the cornea, the iris is a dark, muscular diaphragm. It gives the eye its distinctive colour (e.g., blue eyes, brown eyes) and controls the size of the pupil.
• Pupil: A small opening in the centre of the iris. The amount of light entering the eye is regulated by the size of the pupil, which expands in dim light to allow more light in and contracts in bright light to limit the amount of light.
• Lens: Behind the pupil is a lens, which is thicker in the centre (a convex lens). This lens focuses the light that enters the eye onto the retina.
• Retina: The back surface of the eye is the retina. It is a light-sensitive layer containing millions of nerve cells, of two types:
  • Cones: Sensitive to bright light and responsible for detecting colour.
  • Rods: Sensitive to dim light and responsible for vision in low light conditions.
  When light falls on the rods and cones, they generate electrical signals.
• Optic Nerve: These signals from the retina are transmitted to the brain through the optic nerve. The brain processes these signals to create the visual image we perceive.
• Blind Spot: At the junction where the optic nerve leaves the eye, there are no light-sensitive cells (rods or cones) on the retina. This area is called the blind spot because no vision is possible there. Its existence can be demonstrated by focusing on a mark while another mark disappears from view at a certain distance.

The eye is capable of focusing on objects at different distances. The ciliary muscles surrounding the lens adjust its shape to focus light from nearby or distant objects onto the retina. For a normal, healthy eye, the most comfortable distance for reading is about 25 cm.

Vision defects, such as being able to see nearby objects clearly but distant ones blurrily (myopia) or vice versa (hyperopia), can often be corrected using appropriate corrective lenses (spectacles or contact lenses).

In some cases, often with age, the eye lens can become cloudy or opaque, a condition called cataract. This leads to blurry or significantly reduced vision. Cataracts can be treated by surgically removing the clouded natural lens and replacing it with an artificial lens.

Another interesting property of vision is the persistence of vision. The impression of an image on the retina lasts for a short duration (about 1/16th of a second) even after the light source is removed. If a series of still images are shown rapidly (faster than 16 frames per second), our brain perceives continuous motion. This principle is used in movies and animations.

Nature also provides protection for the eyes through eyelids, which shield them from external objects and regulate the amount of light entering.

Care Of The Eyes

Taking proper care of our eyes is very important to maintain good vision. Consulting an eye specialist for regular checkups is advisable, especially if any problems arise.

Key practices for eye care include:

• Wearing suitable spectacles if advised by a doctor.
• Avoiding exposure to extremely dim or excessively bright light. Both can strain the eyes and cause discomfort. Looking directly at bright light sources like the Sun, powerful lamps, or laser torches can permanently damage the retina.
• Never rubbing your eyes vigorously. If dust or particles enter the eye, wash them with clean water. Seek medical help if irritation persists.
• Maintaining the correct reading distance (approximately 25 cm) and avoiding reading with the book too close or too far.
• Ensuring a diet rich in Vitamin A. Deficiency of Vitamin A is a common cause of eye problems, particularly night blindness (difficulty seeing in dim light). Foods rich in Vitamin A include raw carrots, broccoli, green leafy vegetables (like spinach), cod liver oil, eggs, milk, curd, cheese, butter, and fruits like papaya and mango.

Animals have adapted eyes suited to their lifestyles. Nocturnal animals like owls have large corneas and pupils, and a high concentration of rods in their retina, enabling excellent vision in low light. Diurnal animals like kites and eagles have more cones for good vision and colour perception in daylight.

Visually Impaired Persons Can Read And Write

Some individuals, including children, are visually impaired, meaning they have limited or no vision. This can be due to congenital conditions, diseases, or injuries. Visually impaired individuals often develop their other senses, like touch and hearing, more acutely to interact with their environment. Special resources and techniques are available to help them read, write, and develop their full potential.

What Is The Braille System?

The most widely used method for visually challenged persons to read and write is the Braille system. It was developed by Louis Braille, who was himself visually impaired.

The Braille system uses patterns of raised dots that can be felt with the fingertips. Each pattern, called a character, represents a letter, a combination of letters, a common word, or a grammatical sign. Braille characters are formed within a grid of two vertical rows and three dots each (a cell).

There are 63 possible dot patterns in the Braille system, allowing representation of letters, numbers, punctuation, and words in various languages, including many Indian languages. Visually impaired individuals learn to read Braille by touching and recognising these raised dot patterns. Braille texts can be produced manually using a stylus and slate or by machines, including specialised typewriters and printers.

Technological aids, both non-optical and optical, further assist visually impaired persons. Non-optical aids use senses other than sight (touch, hearing) or electronic assistance (talking calculators, screen readers). Optical aids include various types of lenses and magnifiers to improve remaining vision.

Many visually impaired individuals have achieved great success in various fields, demonstrating that visual impairment does not prevent individuals from leading fulfilling and accomplished lives with determination and appropriate support.

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer: